From its chemical structure, compound I can also be defined as a melatonin derivative or 6-methoxymelatonin. Melatonin is a main secretory product of pineal gland in vertebrates. The synthesis and secretion of melatonin shows a significant circadian rhythm and its production decreases with age. Thus, blood melatonin concentrations are 5-15 times higher at night than during the day and this circadian rhythms deteriorates with aging. Melatonin is a highly conserved molecule during evolution. It is found in most organisms from unicells to humans. Melatonin participates in several important physiological functions including the control of seasonal reproduction, improving immunological function, promoting sleeping, cancer inhibition and anti-aging. Recently, it was found that melatonin is a potent endogenous free radical scavenger (Tan et al., Endocrine J. 1: 57-60 1993). Melatonin may prolong the experimental animal life span up to 27% (Pierpaoli and Regelson, Proc. Natl. Acad. Sci. USA 91: 787-791 1994) and protect neurons against damage induced by a variety of oxidative stresses (Reiter, Prog. Neurobiol. 56: 356-384 1998). The action structure, in terms of free radical scavenging function, is attributed to the 5-methoxy group on the indole ring of melatonin (Tan et al., Endocrine J. 1: 57-60 1993). Direct free radical scavenging ability is a chemical reaction that does not require the molecule to interact with a receptor. When melatonin is used as a free radical scavenger, the receptor-mediated effects would produce side effects, for example, promoting steepness in the day time. Another disadvantage of melatonin is that a major portion of the melatonin in the circulation is quickly metabolized to 6-hydroxymelatonin and excreted into the urine. Melatonin's metabolism to 6-hydroxymelatonin occurs in the liver by a specific enzyme which converts the hydrogen atom at 6 position of indole ring in the melatonin molecule to a hydroxyl group. Thus, the biological half life of melatonin in the blood is short (about 20-40 min). The short biological half life of melatonin makes it less convenient for clinical use as a free radical scavenger (requiring repeated administration of melatonin to keep an effective blood concentration). In order to 1) enhance the free radical scavenging ability; 2) prolong the biological half life and 3) minimize the side effects of melatonin, compound I was designed by substituting the hydrogen atom at the 6 position of indole ring of melatonin with a methoxy group. The methoxy group possesses a p electron and it can share this p electron with the indole ring to form the p-.pi. electron cloud. Once the hydrogen atom at the 6 position of the indole ring of melatonin is substituted with a methoxy group, the intensity of electron cloud on the indole ring is increased and so the free radical scavenging ability correspondingly increases for compound I. Because 6 position of the indole ring in melatonin molecule is occupied by a much larger methoxy group (compared with the hydrogen atom) in compound I the metabolic action of the sterol specific enzyme in the liver is blocked and the degradation rate for compound I is decreased markedly. In this way the biological half life of compound I will be much longer than that of melatonin. Furthermore, the structural change in compound I reduces its binding affinity to the membrane melatonin receptors thereby reducing the side effects of the molecule. To identify these advantages generated by the structural change which were designed by the inventors, both in vitro and in vivo experiments have been employed to test the free radical scavenging activity of compound I. The experimental results proved the prediction.